This invention relates to air filtration systems and, in particular, air filtration systems for clean rooms used for manufacturing processes which require an extremely clean environment.
Certain manufacturing processes, including the manufacturer of semiconductor devices, require an extremely clean environment. This environment is commonly referred to as a clean room. Proper filtering of the air in a clean room is critical in the manufacturing of, for example, semiconductor devices because even very small contaminants can cause decreases in yield and device performance.
Clean room contaminants may be generally classified as either particulate or gas-phase. Common particulate contaminants include dust, lint and other debris. Gas-phase (or molecular-level) contaminants are typically tens-of-thousands of times smaller than particulate contaminants. Because it is now known that gas-phase contaminants are especially detrimental to device yields and device performance of today's increasingly small semiconductor devices, more effort is now being directed to reducing the level of gas-phase contaminants in clean rooms.
Various filters are known in the art for removing gas-phase contaminants from air. Examples of such chemical filters are discussed in U.S. Pat. No. 5,626,820. One such example of a chemical filter is a chemical filter of the pleated filter type comprising an air permeable, relatively thick web of non-woven fibrous carrier material of pleated form. The web includes a matrix formed of a large multiplicity of synthetic fibers and is characterized in that activated carbon particles are distributed throughout the web, bound in the interstices of the matrix in a manner preventing loss to the air of particles in quantity substantially detrimental to the performance of any HEPA filter that may be downstream of the chemical filter. The activated carbon particles are of the type selected to remove predetermined gas-phase contaminants from the air source. Various clean room filtering systems have been developed including the use of chemical filters to reduce gas-phase contamination of clean room air. Examples of such air filtering systems are shown in U.S. Pat. Nos. 5,109,916, 5,626,820 and 5,752,985.
FIG. 1 shows a conventional clean room air filtering system in which a clean room 10 is supplied with filtered make-up air 80 by a make-up air handling unit 50 which filters make-up air supply 70 with make-up air filter 60. The clean room 10 is also supplied with filtered recirculation air 90 by recirculation air handling unit 48. Recirculation air handling unit 48 draws pre-filtered recirculation air 130 across recirculation chemical filter 110 to produce chemical-filtered recirculation air 100. The system shown in FIG. 1 chemical-filters only pre-filtered recirculation air 130 and does not chemical-filter filtered make-up air 80 until after filtered make-up air 80 passes through clean room 10. Clean room filters 20 are provided for additional filtration prior to clean room supply air entering the clean room area. The clean room filters 20 can be, for example, ultra low penetration air (ULPA) filters or other comparable particulate air filters. Plenum 30 receives filtered make-up air 80 and filtered recirculation air 90 and is located adjacent the clean room filters 20.
FIG. 2 shows a conventional clean room air filtering system similar to the filtering system shown in FIG. 1 except that the chemical-filtering is performed on make-up air supply 70 by make-up chemical filter 120 instead of on pre-filtered recirculation air 130 by recirculation chemical filter 110. Chemical-filtered make-up air 85 is supplied to clean room 10 by make-up air handling unit 50.
While the use of chemical filters in clean room air filtering systems for the purpose of reducing gas-phase contaminants may be known in the art, certain problems exist with these systems. For example, some systems, such as the system shown in U.S. Pat. No. 5,752,985, pass only a portion of the air entering the clean room through a chemical filter and, therefore, allow air that has not been passed through a chemical filter to enter the clean room. Similarly, U.S. Pat. No. 5,109,916 shows filtering system in which only a portion of the air entering a conditioned space is chemical-filtered. Also, systems such as the system shown in U.S. Pat. No. 5,752,985 and Japanese Patent Publication No. JP-A-8-89747 use multiple chemical filters, each with its own fan unit. This type of system typically requires a large number of chemical filters and fan units, resulting in a high initial cost and high maintenance costs.
Other clean room filtering systems, such as Japanese Patent Publication No. JP-A-8-89747, locate the chemical filters and fan units in the ceiling space above the clean room. Such systems result in high maintenance costs due to the difficulty associated with accessing the ceiling space as compared to accessing a remotely located chemical filter and fan unit.